Fix crash when dialplan remove include is called with too few arguments.

Fix crash when dialplan remove include is called with too few arguments.

"dialplan remove include x from y" crashed when the amount of arguments
was less than 6.

(closes issue ASTERISK-18762)
Reported by: Andrey Solovyev
Tested by: Andrey Solovyev
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Merged revisions 343936 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 343944 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@343951 65c4cc65-6c06-0410-ace0-fbb531ad65f3